1. Summary of the Invention
Nowadays, especially in Europe and America, as the tendency of producing materials in factories is progressing, the materials which need much labor on the site are regarded as not up to date. In this respect, an architectural precast concrete panel, which plays an important role in architectural designs, has attracted public attention, and reconsideration has arisen to make these panels individual design materials. For a period of time after the 2nd World War, it was said that the surface of concrete itself was beautiful. However, in the future, the application of precast architectural materials will become important. For the development of said architectural materials, there are many problems to be studied, and the technical development is very significant. Especially, with increase of precast concrete building, research for manufacturing technique and construction method has progressed. Further, referring to the finishing procedure for the surface, which is in the scope of the technique of the plasterer, technical progress has decreased the defects in the standardization of surface finishing, and the improvement in mass effect, the variety of material and the quality thereof is remarkable.
2. The Prior Art
However, the usual procedures for finishing of the surface of the precast concrete panel are (a) finishing without any treatment, (b) finishing by exposing aggregates on the surface, (c) finishing with tiling, (d) finishing with adhesion treatment, (e) paint finishing, and (f) finishing of surface treatment by chemical agents and the treatment of surface by carbon dioxide gas. These finishings are carried out usually by employing a secondary procedure, for example by painting, washing with acids, and using surface retarders on the surface, before the hardening of a concrete panel. Therefore, these procedures would not be productive, and for the use of a long period, repairs and inspections are necessary at least once every several years. Considering in general the above matters, there are many problems for the productivity, economical factors and qualities.
More recently, efforts have been made to improve the previous methods, or to develop a novel method, of decorating walls, floors and ceilings of a construction. On this line, processes for producing a novel precast concrete panel having a decorative surface layer from a synthetic resinous binder and aggregates have been developed, some of which can be seen, for example, in the specifications of U.S. Pat. Nos. 3,097,080; 3,439,076; 3,574,801 and 3,632,725.
However, in the processes disclosed in the specifications of the above-identified U.S. Patents, the decorative layers are composed from unsaturated polyester or epoxy resins used as binders for the aggregates. Since these resins are essentially weak in resistance against weathering, they are not suited to materials for construction on account of lack of long enduring stability. Further, those panels which are produced in the disclosed processes are not provided with properties suited to wall, floor or ceiling material themselves. They are intended to be used as applied to the surface of constructions, and are hardly useful materials from the point of view of economy and workability.
Therefore, for production of cheaper, more durable and superior architectural precast concrete panels, the inventors have found that the following conditions are critical and necessary:
1. The process of making the panels should include simultaneous formation of a decorative surface and a concrete layer without any secondary procedure.
2. The physical properties of said concrete panel should not be influenced badly by the simultaneous formation into one body.
3. The durability of said concrete panel should be superior for chemical and physical changes.
4. Repairs should not be necessary for a long period,
5. The decorative surface should be free from efflorescence.
We have investigated how to produce architectural precast concrete panels which meet the above critical conditions and requirements. Firstly, use of an epoxy or an unsaturated polyester resin as a binder was found unsuited to produce a colorless transparent and weather-resistant surface that could be exposed to the weather for a long period without losing stability against weathering. In the next place, use of methyl methacrylate polymer known as organic glass as being colorless, transparent and highly weather resistant was studied. This polymer is described in Cement and Concrete Research, Vol. 1, No. 1, pages 187-210 (1971) where it is stated that when the monomer of methyl methacrylate and a polymerization initiator are mixed with a hydraulic cement composition and the hydration and polymerization reactions are brought to completion, the methyl methacrylate monomer interferes with the hydration reaction of cement and causes deterioration of the physical properties of the concrete or mortar products.
However, we have succeeded in producing a sufficiently strong adhesive power between the decorative layer, consisting of methyl methacrylate resin and supporting material, and the lining concrete layer only by using partly polymerized methyl methacrylate monomer in syrup in combination with the radical-polymerizing cross-linking monomer. In addition, the transparency of the methyl methacrylate resin permits the three-dimensional appearance of the supporting material to be fully realized. The product satisfies all the above listed critical requirements of precast concrete panels. Thus, we have accomplished the present invention.